LPG vs Liquid Fuels

On last week’s show I was extolling the virtues of diesel (modern turbo diesels) over petrol.

Someone called up off air and said “you convinced me of diesel, but what about LPG?”

It’s a good question.  LPG of course stands for liquefied petroleum gas, and is actually a mixture of butane and propane.  When it’s pressurised it turns into a liquid, and that’s the liquid you see sloshing around inside a cigarette lighter.

There are several advantages of LPG over both petrol and diesel.  The most obvious one, of course, is that its much cheaper.  But there are other advantages.

Most notable, of course, is the fact that the motors run so clean.  The reason for this is quite straightforward.  Simply put, as soon as it leaves the fuel tank it is already a gas.  Both petrol and diesel, on the other hand, are of course liquids, and the fuel delivery system on your car, whether it be a carburettor or fuel injectors, has the job of doing its level best to turn that liquid into a gas.

Unfortunately, this process is less than perfect, particularly on the older type diesels, and the consequence of this is that your engine gets some liquid fuel that doesn’t burn and runs down into the sump.

Now, as we have seen, the combustion of fuels is less than perfect, with one of the products being carbon.  The most likely place for this to form is on the surface of one of the liquid fuel droplets, as the inside of the drop would not contain any oxygen, so it’s the right conditions for the oxygen starvation that’s will result in carbon, instead of CO2.

Because it forms in the drop, it then falls into your sump and hence the dirty oil.

But with LPG since it is a gas, the tiny particles of carbon that may form just get blown out of the engine..

So LPG is a great choice from that standpoint.  The only disadvantage it has, is that if you run out of fuel, it’s a towing job to refill.  If you’re going for a drive in the country, a second problem emerges – it’s not that easy to find service stations that sell it.

But one of the things that puzzles me most about LPG is that so few car manufacturers make it as a factory option.  The only attempt at a dedicated LPG vehicle of course is the Falcon, but it was a pretty poor attempt.  But I don’t understand why other manufacturers paint make dedicated LPG vehicles – I reckon they’d sell like hotcakes.

Removing Tyre Marks

Occasionally I get asked how to remove tyre stains.

The ease with which this process can be done depends on what they are on.  If it’s a hard surface like pavers, cement, or tiles, it’s easy.  All you need is heat.  The hard surface will not be affected by the heat, but the rubber will.  It will probably go black at first but then finally disappear as it is all burned off.

This is simply called “high temperature incineration,” and is a common way of removing waste.  All the carbon is converted to CO2 and all sulphates and nitrates are converted to SO2 and NO2 respectively.

So all you need is a heat gun, or if that isn’t hot enough (it probably is) you could use a butane torch.

If the tyre marks are on the bitumen road, however, there is nothing you can do, as bitumen is very soft and any attempt to remove the rubber will also damage the bitumen.  But the good news is that because the bitumen is black, or least grey, the tyre marks will eventually fade and disappear from view.

Petrol and Safety

Petrol is probably the most dangerous chemical that most people will encounter from day to day.

This is because it is both carcinogenic and flammable.  The carcinogenicity is mainly the result of benzene which is such a common chemical in crude oil, it would be both impractical and uneconomical to attempt to remove it.

Probably a bigger problem however is its flammability.

For years the myth has been around that mobile phones can ignite petrol.  There have been reports floating around the Internet to be effect that Shell has issued an official warning about using mobile phones whilst filling your car with petrol.

As it happens, these reports are myths.  I’ve searched high and low but have been unable to find any such reports directly from Shell.  And, of course, mythbusters famously dealt with this myth.  It must be said, however, that the myth busters treatment was not conclusive.  This is simply because they would have to have tested every mobile phone in existence before they could conclusively say that a mobile phone could not be an ignition source.

Certainly, if it was ever true, it was only true of the older, higher powered analogue phones that have now disappeared.

In any case however there is a much more real danger posed by the flammability of petrol – static electricity.

When a metal object moves through the air, it picks up static electricity.  Like cars, planes, and zeppelins (but that’s another story).

So when you pull up at a service station and hop out of your car, it is holding a charge because it is electrically insulated from the earth by its rubber tyres.  And you are holding a charge as well.

When you put the nozzle into the car, this earths the car.  And it sure would earth you as well.  But if you hop back into the car, particularly if it is a dry day and you are wearing synthetics like nylon, the action of you rubbing against the car seat can restore static electricity to your body.

If you then go back and out to a running pump, like this girl did, a spark can jump between you and the petrol nozzle if that’s the first thing you touch, with predictable results.

So the lesson is, if you hop back into your car while you are refuelling, when you hop out of the car touch your hand against the side of the car to earth your body before you go anywhere near the fuel pump.

But of course, none of this is an issue with diesel, as it is non-flammable.

Petrol vs Diesel #4: Hybrids

We have seen that for a given engine size a modern turbodiesel produces more torque lower in the rev range than a petrol motor, and uses a lot less fuel.

Whenever this is raised with the petrol fraternity, the comeback is always “well, get a hybrid then.”

Let’s have a look at it – how does a state of the art petrol hybrid motor stack up against a modern turbodiesel motor.  We will compare the Toyota Prius with the Ford Fiesta diesel.

But first, let’s look at the concept of the hybrid –  what are they, and what are they supposed to do.

It seems to me that most people think that a hybrid motor is somehow able to pull energy out of the air, and deliver spectacular fuel economy as a result.

Let’s get one thing clear – the ultimate source of energy in your car is the very molecules in your fuel, and the chemical energy they contain, whether it be a petrol hybrid or a regular motor.

The hybrid improves economy by one thing and one thing alone – it minimises the energy losses that result in city driving with many stops and starts.  That’s all a hybrid is – its ultimate goal is to make the fuel economy as close as possible to the economy you would get with country driving.  We all know that for most cars there is a significant difference between the fuel economy you get in the city and the country.

The reason simply is that when you brake you are taking energy out of the system, and you must put it back in by accelerating, therefore using extra fuel – more than you would if you hadn’t had to brake.  When you brake the mechanical energy of the car is converted into mechanical action (scraping away your disks and brake pads) and heat.

With the hybrid, however, when you brake a server kicks in which acts to charge the batteries.  That is, the energy is not lost – it is put back into your batteries, to be used again at a later time.  Those of us who are a little older will remember the old pushbike generators.  When you turned the generator on so that you had lights, your bike slowed down, and you had to do extra work to turn the generator to provide the light.

The brakes in hybrids work the same way. Although they contain regular disks and pads, these are only used in more abrupt stopping, and for less abrupt stopping a dynamo generates power that goes back into your batteries.

So the idea is that the fuel consumption of a hybrid is pretty much the same in the country as the city.  And this is certainly the case with the Prius.  Country driving uses 3.7 L/100km  and city driving is 3.9 – hardly any difference at all.  This therefore results in a lower average (quoted at 3.9).

Let’s now look at the Fiesta diesel.  Its country economy  is 3.2 L/100km and its city economy is 4.6.  This kind of variation is normal in non-hybrid vehicles.  The quoted average however is 3.7, lower than the hybrid.

Also the Fiesta producers 200 Nm of torque at only 1750 rpm, whereas the hybrid has to spin up to 4000 rpm to produce its maximum of 142 Nm.

And then there’s the cost – the Fiesta comes in at 22K, whereas the Prius starts at 33K, going up to 50 K.

On top of this, the batteries in hybrid vehicles do not last as long as the motors, and must be replaced at certain intervals.

And in case you aren’t convinced yet, there is one last advantage of diesel over petrols.  Service intervals.  One of the reasons that the oil in your motor must be changed is because unburnt fuel gets into the oil and dilutes it, so it loses its ability to lubricate your engine.  In years gone past this has been a bigger problem in diesels then petrol, as diesel is less volatile than petrol, and will not evaporate as quickly.

But now the situation is the opposite.  Modern turbodiesels are so incredibly efficient that there is virtually no oil pollution happening.  Therefore, they have extraordinarily long service intervals – up to 30,000 km for some European vehicles.

So let’s summarise.

Diesel motors last longer, have longer service intervals, produce more talk at lower revs, and use less fuel.  Also diesel as a chemical is safer than petrol.

Diesel or petrol?  It’s a no-brainer.